BBC NEWS | Science/Nature | Astronaut pair leaves ISS empty: "Commander Foale has said he is happy to carry out the five-and-a-half-hour walk, recalling that pairs of Apollo astronauts had made lengthy walks across the Moon's surface in the 1960s and 1970s without anyone staying behind on their lander. "

Well, that's certainly true but I don't think there was much risk that the lander was going to float away on its own or that they might get locked out of it. Walking across the surface of the moon, while significant of course, is not quite the same thing as doing a spacewalk in open space.

Or am I just displaying my ignorance of the operational details of space walking?

Update: The risky spacewalk had to be cut short due to a problem with one of their flight suits.